The cubic semiconducting compounds APd 3 O 4 (A = Ca, Sr) can be hole-doped by Na substitution on the A site and driven towards more conducting states. This process has been followed here by a number of experimental techniques in order to understand the evolution of electronic properties. While an insulator-to-metal transition is observed in Ca 1−x Na x Pd 3 O 4 for x ≥ 0.15, bulk metallic behavior is not observed for Sr 1−x Na x Pd 3 O 4 up to x = 0.20. Given the very similar crystal and (calculated) electronic structures of the two materials, the distinct behavior is a matter of interest.We present evidence of local disorder in the A = Sr materials through the analysis of the neutron pair distribution function which is potentially at the heart of the distinct behavior. Solid-state 23 Na nuclear magnetic resonance studies additionally suggest a percolative insulator-to-metal transition mechanism wherein presumably small regions with a signal resembling metallic NaPd 3 O 4 form almost immediately upon Na substitution, and this signal grows monotonically with substitution. Some signatures of increased local disorder and a propensity for Na clustering are seen in the A = Sr compounds.

Quantum information science and engineering require novel low-loss magnetic materials for magnon-based quantum-coherent operations. The search for low-loss magnetic materials, traditionally driven by applications in microwave electronics near room temperature, has gained additional constraints from the need to operate at cryogenic temperatures for many applications in quantum information science and technology. Whereas yttrium iron garnet (YIG) has been the material of choice for decades, the emergence of molecule-based materials with robust magnetism and ultra-low damping has opened new avenues for exploration. Specifically, thin films of vanadium tetracyanoethylene (V[TCNE]x) can be patterned into the multiple, connected structures needed for hybrid quantum elements and have shown room-temperature Gilbert damping (α = 4 × 10−5) that rivals the intrinsic (bulk) damping otherwise seen only in highly polished YIG spheres (far more challenging to integrate into arrays). Here, the authors present a comprehensive and systematic study of the low-temperature magnetization dynamics for V[TCNE]x thin films, with implications for their application in quantum systems. These studies reveal a temperature-driven, strain-dependent magnetic anisotropy that compensates the thin-film shape anisotropy and the recovery of a magnetic resonance linewidth at 5 K that is comparable to room-temperature values (roughly 2 G at 9.4 GHz). The authors can account for these variations of the V[TCNE]x linewidth within the context of scattering from very dilute paramagnetic impurities and anticipate additional linewidth narrowing as the temperature is further reduced.

Paramagnetic magnetic resonance, a powerful technique for characterizing and identifying chemical targets, is increasingly used for imaging; however, low spin polarization at room temperature and moderate magnetic fields poses challenges for detecting small numbers of spins. In this work, we use fluorescence from nitrogen-vacancy (NV) centers in diamond to detect the electron paramagnetic resonance (EPR) spectrum of optically inactive target spins under various conditions of field magnitude and orientation. The protocol requires neither direct microwave manipulation of the NV spins nor spectral overlap between NV and target spin resonances, thus enabling broadband detection. This unexpected non-resonant coupling is attributable to a two-phonon process that relaxes NV spins proximate to the fluctuating dipole moment of the target spin, suggesting that the sensitivity is determined by the dipoledipole coupling strength. This approach holds promise for sensitive EPR detection, particularly in settings where control over the diamond-crystal orientation is difficult. This is notably the case for biological sensing applications, where nanodiamonds are being pursued for their bright, stable fluorescence and biocompatibility.

We report on micro-focused Raman spectroscopy of encapsulated vanadium tetracyanoethylene (V­[TCNE] x , x ≈ 2) films as they age under ambient conditions to understand the structural changes that accompany aging and how those changes influence the magnetic properties. V­[TCNE] x is an organic-based ferrimagnet with a high magnetic ordering temperature T C > 600 K, low magnetic damping, and growth compatibility with a wide variety of substrates. However, similar to other organic-based materials, it is sensitive to air. Although encapsulation of V­[TCNE] x with glass and epoxy extends the film lifetime from hours to weeks, its aging processes remain poorly understood. We identify the relevant features in the Raman spectra in agreement with ab initio theory, reproducing CC and CN stretching vibrational modes. We correlate changes in the Raman intensity and in photoluminescence to changes in the magnetic properties of the sample as measured using ferromagnetic resonance and magnetometry. Based on changes in the Raman spectra, we hypothesize structural changes and aging mechanisms in V­[TCNE] x . We also find that we can introduce similar changes using focused laser illumination at high intensity, enabling a new mechanism for “erasing” magnetism in this material through selective modification of local bonding. These findings enable a local optical probe of V­[TCNE] x film quality, which is invaluable in experiments where assessing film quality with local magnetic characterization is not possible, and they enable patterning of V­[TCNE] x by laser writing.

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